The present invention is generally directed to fuel cells and more specifically to a cascaded fuel cell stack system.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. There are classes of fuel cells, such as the solid oxide reversible fuel cells, that also allow reversed operation.
In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow is typically a hydrogen-rich gas created by reforming a hydrocarbon fuel source. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
Fuel cell stacks may be either internally or externally manifolded for fuel and air. In internally manifolded stacks, the fuel and air is distributed to each cell using risers contained within the stack. In other words, the gas flows through riser openings or holes in the supporting layer of each cell, such as the electrolyte layer, for example. In externally manifolded stacks, the stack is open on the fuel and air inlet and outlet sides, and the fuel and air are introduced and collected independently of the stack hardware. For example, the inlet and outlet fuel and air flow in separate channels between the stack and the manifold housing in which the stack is located.
Planar fuel cells are usually stacked on top of one another with the stacking direction perpendicular to the planar cell elements thereby forming an electrical series connection. Any fuel cell stack requires a supply of fuel and oxidizer as well as depleted reactant removal (which usually entails two fluid streams, but these two streams may be combined into a single stream). In a power generator containing a multitude of fuel cell stacks, the reactant fluid delivery and collection to/from these stacks can create significant complexity. Therefore it is often desirable to minimize the number of stacks which may be accomplished by stacking more cells into each stack.
However, the number of cells that can be operated in a single stack is often limited by the reactant fluid, such as fuel gas, flow to the individual cells. The fluids distribution plenum that delivers the reactant fluids to the individual cells imparts a pressure drop on the fluid flow. A taller stack implies a larger pressure drop in the distribution plenum. One consequence of increasing pressure drop is that the fluid movers in the system (pumps, blowers, or compressors) consume more energy. Another consequence of the pressure drop in the fluids distribution plenum is that cells in different regions of the stack may experience different fluid flow rates.
In a stack with identical repeating elements, the performance is limited by the weakest cell. Cells with lower reactant fluid flow rates usually perform worse than cells with higher flow rates. The fuel supply is usually kept high enough so the cell experiencing the least fuel flow can still fulfill the performance requirements of the stack.
In a stack, all cells are usually operated electrically in series. Therefore all cells operate at the same current and all cells consume the same amount of fluids. Excess fuel supplied to the cell either has to be recovered and recycled or contributes to losses in the system.
High efficiency systems strive to minimize the cell to cell flow differences and maximize the attainable fuel utilization. FIG. 1 shows a typical internally manifolded fuel cell stack 1 comprising a plurality of fuel cells 3 in which the fuel is distributed within the stack in the so called risers or riser channels, which are formed by aligned openings in all electrolytes (and/or electrodes in electrode supported cells) and interconnects. In FIG. 1, fuel is supplied upward through the fuel inlet riser 5 and depleted fuel (i.e., the fuel exhaust) is removed downward through the fuel exhaust riser 7. If this stack is built very tall, the upper cells 3 will experience less fuel flow than the lower cells 3. Therefore, the total fuel flow rate to the stack has to be increased to provide enough fuel to the cells with the lowest flow (i.e. upper cells), while cells at the bottom of the stack are supplied with excess fuel. This excess fuel contributes to a loss of efficiency, unless the unused fuel is recycled.
Furthermore, solid oxide fuel cell stack columns can develop relatively large thermal gradients due to their configuration. For instance, the middle of two hundred cell stack column may be 100° C. greater than areas at the ends of the column. Such thermal gradients can affect the operation of the fuel cell stack because cells within the stack are not operating at an optimum temperature or within an optimum temperature range. For example, cells operating at a temperature that is lower than an optimum temperature can have a higher internal impedance, resulting in a lower voltage, and cells operating at a temperature that is higher than an optimum temperature can be subjected to higher rates of material degradation, causing the hotter cells to experience shorter cell lifetimes.